This invention relates to a semiconductor light emitting device which is improved in semiconductor crystalline quality to have an enhanced light emitting efficiency, for use as a light source requiring high brightness, such as for outdoor displays and vehicular tail lamps and direction indicators, or a light source requiring high brightness but low-power consumption, such as for back-lights of battery-driven portable units such as handy telephones and indicator lamps, besides for optical communications or optical information processing.
There is a conventional semiconductor light emitting device having a light emitting layer forming portion formed by using an AlGaInP-based compound semiconductor for emitting visible light, as disclosed, for example, in Japanese Laying-open Patent Publication No. H4-212479. This known semiconductor light emitting device is structured as shown in FIG. 4. In FIG. 4, the device includes an n-GaAs semiconductor substrate 21. On the semiconductor substrate 21 are epitaxially grown, in order, an n-type cladding layer 22 of an n-type AlGaInP-based compound semiconductor, an active layer 23 of a non-doped AlGaInP-based semiconductor material in a composition having a bandgap energy lower than that of the cladding layer, and a p-type cladding layer 24 of a p-type AlGaInP-based compound semiconductor, thereby forming a doublehetero structure providing a light emitting layer forming portion 29. Further, a p-type window layer (current diffusion layer) 25 is epitaxially grown of a GaAs-based semiconductor material on a surface of the light emitting layer forming portion 29. On the window layer 25 a p-side electrode 27 is formed through a p-type GaAs contact layer 26, while an n-side electrode 28 is formed on a backside of the semiconductor substrate 21. These electrodes 27, 28 are formed of an Au--Ge--Ni alloy or the like.
The light emitting device of this structure is arranged to confine carriers within the active layer 23 sandwiched between the respective cladding layers 22, 24, for emitting light. Accordingly, the cladding layers 22, 24 and the window layer 25 are doped with an impurity to an appropriate carrier concentration. The p-type layers 24, 25, 26 are doped with an impurity such as Zn, Mg or Be. Meanwhile, the window layer 25 has to be formed of a material having a higher bandgap energy than the bandgap energy of the emission-light wavelength, in order not to absorb the light emitted by the active layer. Thus, the AlGaAs-based compound semiconductor material is used for the window layer 25. Even where the AlGaAs-based compound semiconductor is employed, if it is low in Al mixed-crystal ratio, the bandgap energy is decreased to absorb a certain amount of the light emitted by the light emitting layer forming portion. Accordingly, the compound semiconductor in practical use has an Al mixed-crystal ratio increased to approximately 0.7-0.8.
In the conventional semiconductor light emitting device shown in FIG. 4, the GaAs substrate and the AlGaInP-based compound semiconductor are controlled in lattice-match by adjusting the mixed crystal ratio between (AlGa) and In. Also, the AlGaAs-based compound semiconductor, if doped with Zn or Mg, becomes mismatching in lattice to the AlGaInP-based compound semiconductor. In such a case, a process for further matching is necessarily performed, resulting in poor film crystallinity. On the other hand, it is not preferred for the cladding layer to increase the carrier concentration, in view of the effect of carrier confinement within the active layer and the suppression against diffusion of an impurity from the cladding layer into the active layer. In contrast to this, it is preferred that the window layer be given a carrier concentration as high as possible. However, if the window layer is doped to a high concentration with a p-type impurity (2-valence atom with respect to a III-V compound semiconductor) such as Zn, Mg or Be, the film crystallinity worsens as stated before. This might cause a phenomenon such as cracks or chip fracture in the semiconductor layer due to internal strains or distortions, as can be observed by a reliability test with applying currents at low temperatures. This raises a problem that the device is lowered in light emitting efficiency or tendency to be damaged.
Meanwhile, the emission light is transmitted toward the front of the semiconductor light emitting device, even where a window layer the device is formed of an AlGaInP with an increased Al mixed-crystal ratio on the surface thereof, similarly to the semiconductor light emitting device as shown in FIG. 4. However, there is absorption or blockage of the emission light by the front electrode as well as the GaAs contact layer for improving ohmic-contact characteristics between the electrode and the semiconductor layer. To cope with this, the top electrode and the contact layer is removed away with a required minimum area left, thus exposing the window layer at a top surface. This surface, in some cases, is covered by a package resin or the like. However, the resin and the semiconductor are poorly matched, giving rise to intrusion of moisture or water content through an interface therebetween. Since in such a case the semiconductor material having an increased Al mixed-crystal ratio appears in the interface, the Al content reacts with the intruded moisture, causing corrosion or oxidation. The corrosion or oxidation, if proceeds deep inside the semiconductor layers, possibly deteriorating the crystalline structure of the light emitting layer forming portion. Thus, there is a problem that the device might be degraded in light emitting efficiency as well as reliability.